GPR, or ground-penetrating RADAR (where RADAR is “RAdio Detection And Ranging), is a technology used to assess the composition and location of heterogeneous materials. GPR uses radio frequencies and is particularly useful in that it is both non-destructive and non-ionizing. In fact, GPR uses frequencies similar to a cellular phone, but at far lower power levels. Common applications include locating the precise position of rebar within a concrete wall/floor, identifying and locating buried objects underground, assessing the quality and uniformity of an asphalt or concrete highway surface, and detecting deterioration on bridge decks. In road surface applications, GPR is used, for example, to detect cracks, fissures, or contamination in any one of the chip seal, pavement layers, gravel base, and so forth. In many roadway applications, a resolution of features of the road surface of less than one inch (2.54 cm) is desired. Such systems may be mounted on vehicles traveling over the surface while acquiring measurement data.
Referring now specifically to paving of asphalt surfaces (a mixture of bitumen pitch with sand, gravel, or stone), when asphalt surfaces are built or rehabilitated, the degree of compaction of the newly-laid asphalt is very important. Poorly-compacted asphalt has a lifetime that may be as little as half the lifetime of properly compacted asphalt. Asphalt compaction levels are often measured after the asphalt compaction process is completed to reward or penalize the paving contractor depending on the measured compaction levels. The measurements may be made with ground penetrating radar (GPR) or handheld nuclear or non-nuclear gauges. Jaselskis et al, (U.S. Pat. No. 5,952,561) provide ample discussion on the compaction process and methods comprising the current state-of-the art.
Ground penetrating radar has the potential of being used on the rollers that do the compacting to provide real-time feedback to the roller operator so that the roller operator can determine how many passes with the roller are necessary for optimum compaction. A GPR system that produces real-time dielectric values has been developed and is used to for quality assurance of asphalt roads after they have been compacted. This system can be calibrated based on the asphalt mix used for a particular paving project to output the asphalt density or percent void content in real-time. One issue that limits the accuracy of the GPR measurements, and consequently its deployment on rollers, is the presence of wetness on the asphalt surface. The source of the wetness might be precipitation or the fine spray that soaks the surface of the rollers to prevent the asphalt from sticking to the roller as it is rolling over the hot asphalt. The GPR method uses the amplitude of the reflection of the RADAR waves from the asphalt surface to calculate the asphalt dielectric. The presence of water on the asphalt surface impacts the surface reflection resulting in inaccurate and often non-useful measurements.
Thus, there is a need to be able to determine compaction of the asphalt despite the inaccurate measurements that result from water on the surface thereof.